1. Field of the Invention The present invention relates to an antiferroelectric liquid crystal panel which is a kind of liquid crystal panel used for a display device or a light shutter.
2. Description of the Related Art
Since a liquid crystal display device using an antiferroelectric liquid crystal panel has characteristics of faster response speed and a wider viewing angle compared with a conventional nematic liquid crystal panel, future growth is expected.
The antiferroelectric liquid crystal has the same smectic layer structure as a ferroelectric liquid crystal. In a liquid crystal cell where a cell gap between substrates filled with the antiferroelectric liquid crystal is made 1 to 3 .mu.m, and the liquid crystal molecules are aligned to be parallel to the substrate surface, a switching phenomenon caused by electric field induction is created.
That is, when voltage is not applied, each long axis of the liquid crystal molecules tilts at a predetermined angle to the normal of the layer in the opposite direction to each other, and the directions of the spontaneous polarization created in the direction intersecting at right angles to the long axis of the molecule form an alternate alignment structure facing upward and downward for every layer to make an antiferroelectric liquid crystal panel, and the entire cell becomes an antiferroelectric layer having zero spontaneous polarization.
On the other hand, when a voltage is applied, each liquid crystal molecule turns in a manner that the spontaneous polarization of the liquid crystal molecule aligns in a direction coinciding with an electric field direction caused by the applied voltage and produces a ferroelectric layer whose molecules all tilt in the same direction with respect to the normal direction of the smectic layer. The direction in which the liquid crystal molecules tilt at this moment depends on the polarity of the applied voltage.
A liquid crystal display panel or a liquid crystal shutter can be structured utilizing the switching phenomenon in a layer state caused by the electric field induction of the above-described antiferroelectric liquid crystal, varying a polarization state of transmitted light, and controlling the amount of light transmitted by combining a polarizing film.
Here, a structure of the conventional antiferroelectric liquid crystal panel is explained with reference to a sectional view in FIG. 13.
The antiferroelectric liquid crystal panel 100 has a first substrate 101 and a second substrate 102 which are made of a glass plate, opposedly disposed, and the antiferroelectric liquid crystal 103 is filled in the gap between the substrates.
On each confronting surface of the first substrate 101 and the second substrate 102, a number of strip-shaped transparent electrodes 105 and 106 are aligned to intersect with each other at approximately right angles. Alignment layers 107 and 108 coated with resin film made of polyimide or polyamic acid are formed to cover each transparent electrode 105 and 106, and an aligning treatment performed by a rubbing is applied on each surface.
In order to keep the gap between the first substrate 101 and the second substrate 102 constant, silica beads 109 having a diameter in the range of about 1.5 .mu.m to 1.7 .mu.m are scattered as a spacer between both substrates. The silica beads 109 are scattered at a density in the range of 500 beads/mm.sup.2 to 1000 beads/mm.sup.2 to make the cell gap uniform.
A seal material 110 is formed on the second substrate 102 with a thermosetting adhesive. The first substrate 101 and the second substrate 102 are placed one upon another with correct positioning and by bonding each other with the sealing agent 110, the gap in which the antiferroelectric liquid crystal 103 is filled, is sealed.
The seal material 110 is a thermosetting adhesive patterned along the periphery of the second substrate 102. After placing the first substrate 101 on the second substrate 102 and bonding each to the other, the seal material 110 is hardened by a heating through applying pressure of 1.0 to 2.0 kg/cm.sup.2 and a temperature in the range of 120.degree. C. to 160.degree. C. for one to two hours in an oven.
On the outside of the first substrate 101 and the second substrate 102 of the antiferroelectric liquid crystal panel 100 thus formed, polarizer 111 and 112 are arranged in a manner that each polarization axis intersects at right angles with the other to form a liquid crystal display panel.
In the above-described antiferroelectric liquid crystal panel 100, the presence or absence of the voltage applied on the antiferroelectric liquid crystal can be controlled at each intersecting portion of the first transparent electrode 105 and the second transparent electrode 106, so that an antiferroelectric state and a ferroelectric state of the antiferroelectric liquid crystal are selectively created so as to partially modify the polarization state of light passing through the antiferroelectric liquid crystal panel 100.
Then by disposing a pair of polarizers 111 and 112, of which polarization axes intersect with each other at right angles, on both sides of the antiferroelectric liquid crystal panel 100, various displays can be conducted by controlling the amount of light transmitted at each display pixel, which are the intersecting portions of the first transparent electrode 105 and the second transparent electrode 106.
In such an antiferroelectric liquid crystal panel, the gap between the first substrate 101 and the second substrate 102 is smaller than that of the liquid crystal display panel using a conventional twisted nematic liquid crystal, and is preferably 2 .mu.m or less. Therefore, a silica bead 109 having a diameter in the range of 1.5 .mu.m to 1.7 .mu.m as described above is used as a spacer to keep the gap constant. However, it has such disadvantages as follows.
That is, in a liquid crystal display panel which is made by bonding two sheets of polarizers 111 and 112 on the outer surfaces of the first and second substrates 101 and 102 of the antiferroelectric liquid crystal panel 100 shown in FIG. 13, the silica beads 109 are unavoidably scattered on a display pixel portion (an intersecting portion of the first transparent electrode 105 and the second transparent portion 106). Accordingly, there arises a disadvantage that a void is created due to the restriction on the movement of the liquid crystal molecules by the silica beads 109, or a delicate void transmission of light is created due to the diffracted light around the silica bead.
When the antiferroelectric liquid crystal panel is formed by placing the first substrate 101 upon the second substrate 102, since some shear stress occurs during a positioning process, the silica beads are slightly shifted to create an alignment defect on the alignment layers 107 and 108, and there arises another disadvantage of the lowering of the black level when no voltage is applied.
These disadvantages result in not only lowering the contrast of the liquid crystal display panel using the antiferroelectric liquid crystal panel, but also creating a disadvantage of lowering the reliability in quality due to the alignment failure caused by damaging the alignment layer.
In the case of the antiferroelectric liquid crystal panel, since the gap between the substrates is a narrow gap of less than 2 .mu.m, both substrates slightly vibrate, resonating with the driving frequency of the voltage applied to the antiferroelectric liquid crystal. Since the alignment layer is damaged due to shifting of the silica beads caused by the vibration, there arises still another disadvantage of the aligning force being gradually lowered.
Further, in the antiferroelectric liquid crystal panel, when the liquid crystal display panel is formed by bonding the polarizers 111 and 112 on both sides of the first substrate 101 and the second substrate 102 shown in FIG. 13, since a pressure of more than 5 to 30 g/cm.sup.2 is applied, it is possible to cause damage to a regularly aligned layer structure which is a characteristic of the antiferroelectric liquid crystal molecules due to the applied vertical pressure. This also creates an alignment defect, which results in yet another disadvantage of deterioration in the display quality.
Therefore, the conventional antiferroelectric liquid crystal panel has a disadvantage that it not only requires special equipment for bonding the polarizer film so as not to damage the layer structure, but also it can not be used for a touch panel to enable pen input to a computer from the upper surface of the panel.